The measuring or monitoring of evoked or continuous bioelectric signals in a patient, such as an infant or other human patient who may be incapable of audiometric behavioral responses, is becoming an increasingly common method for initial patient screening or monitoring, and is used in auditory testing programs to identify hearing abnormalities, or in anesthesia and sedation monitoring to determine a patient's state, such as an awareness level.
In auditory screening, it is well known that the functionality of the outer hair cells of the inner ear can be assessed with measurements of sounds in the external ear canal generated by the inner ear, called otoacoustic emissions (OAE). The sounds which are generated by the inner ear in response to a single introduced click are called transient evoked OAE (TEOAE). Sounds in the inner ear which are generated in response to the presentation of two simultaneous tones are called distortion product OAE (DPOAE).
As shown in FIG. 1, a TEOAE is generated in response to a transient test signal, usually a sequence of discrete square waves (clicks). The level of these clicks is typically between 35 dB SPL and 90 dB SPL. In response to these test signals, a normal human ear generates a wide band response signal up to 20 ms in duration after the introduction of each click. As shown in FIG. 1, the spectrum ST of this response can be compared against the spectrum of ambient noise SA to identify normal or abnormal hearing.
Similarly, as shown in FIG. 2, a DPOAE is generated in a human ear in response to the presentation of two simultaneous tonal signals, s1 and s2 with associated frequencies f1, and f2, with f2>f2. Typically, the ratio of the frequency of f2 to f1 is selected to be about 1.2, with amplitudes |s1|=65 dB SPL and |s2|=55 db SPL in the ear canal. In response to these signals, a normal human ear generates, among others, a third tonal signal, the DPOAE at frequency 2f1-f2, which can be measured to identify normal or abnormal hearing.
An alternative method for testing the hearing of a human patient utilizes surface electrodes to detect bioelectric signals in a human patient which are generated in response to the introduction of an auditory stimulus. These bioelectric signals can be used both in auditory screening and in brain activity monitoring during anesthesia or sedation. An auditory evoked potential (AEP) is generated in a human patient upon presentation of an auditory stimulus or series of stimuli, such as clicks or tone bursts. The AEP can be characterized by three components which refer to the latency of the bioelectric signal response with respect to the introduction of the stimulus; these are referred to as early, middle, and late AEP components.
The early or short latency component of the AEP, also known as the auditory brainstem response (ABR), occurs within the first 15 ms after the presentation of the auditory stimulus in the human ear and is widely used for clinical evaluation of hearing in infants and other individuals who are unable to effectively communicate whether a sound was detected. In individuals with normal hearing, the ABR generates a characteristic neural waveform shown in FIG. 3.
Auditory testing using the ABR typically involves a visual or statistical comparison of a tested individual's waveform to a normal template waveform. Like other evoked potentials, the ABR is recorded from surface electrodes on the scalp. However, the electrodes also record the background noise comprised of unwanted bio-potentials resulting from other neural activity, muscle activity, and unwanted non-physiological sources in the environment.
The middle component of the AEP, the auditory mid-latency response (AMLR), also referred to as the middle latency auditory evoked potential (MLAEP) occurs 15 ms-100 ms after the presentation of the auditory stimulus to the human patient, and is believed to reflect primary, non-cognitive, cortical processing of auditory stimuli. Lately, the AMLR, or MLAEP, has been of particular interest as a measure of depth of anesthesia.
It is known that the AMLR consists of positive and negative waves that are sensitive to sedatives and anesthetics. In general, increasing the level of sedation or anesthetic increases the latency of these waves, and simultaneously decreases the amplitudes. For monitoring purposes, changes in the AMLR waves are quantified as latency to peak, amplitude, and rate of change, and are sometimes combined in a single index.
Another component of the AEP, the auditory late response (ALR) occurs about 100 ms after the introduction of auditory stimulus to the human patient, and is believed to be especially sensitive to the level of sedation or anesthesia applied to a patient, and exhibits a distinct flattening of the waveform at a relatively light level of sedation or anesthesia, among other features.
It is further known that a 40 Hz auditory signal can induce an enhanced “steady-state” AEP response signal in a human patient. Conventional signal averaging over a period of time is required to extract the AEP signal from background EEG signals, and adequate responses usually may be obtainable in about 30-40 seconds. The existence of an intact AEP is believed to be a highly specific indicator for the awake state of a patient, and gradual changes in the depth of sedation or anesthesia appear to be reflected by corresponding gradual changes in the AEP.
Several methods of encoding conventional signals for transmission and reception are known which provide a resistance to signal noise. For transmitted and received signals there are two variables, frequency and time. Division by frequency, so that each pair of communicators (transmitter and receiver) is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access (FDMA). Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access (TDMA). In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time. CDMA uses codes to identify connections. In this transmission technique, the frequency spectrum of a data-signal is spread using a code uncorrelated with that signal. As a result the bandwidth occupancy is much higher then required.
Code Division Multiple Access uses unique spreading codes to spread the baseband data before transmission. The signal is transmitted in a channel, which is below noise level. The receiver then uses a correlator to despread the wanted signal, which is passed through a narrow bandpass filter. Unwanted signals or noise will not be despread and will not pass through the filter. Spreading codes take the form of a carefully designed one/zero sequence produced at a much higher rate than that of the baseband data. The rate of a spreading code is referred to as chip rate rather than bit rate.
Accordingly, it would be advantageous to provide a method and apparatus for utilizing the benefits of a coded signal transmission and corresponding coded response reception to enhance the performance of medical testing devices adapted to introduce an auditory signal to evoke a response, and to measure bio-potentials such as auditory evoked potentials and auditory brainstem response signals.